The goal of this proposal is to develop a toolbox of genetically encoded indicators for biogenic amines, the most important family of neuromodulators. All nervous systems are subject to neuromodulation, which reconfigure the dynamics of neural circuitry by transforming the intrinsic firing properties of targeted neurons and regulating their synaptic plasticity. The altered dynamics of the neuromodulators have been implicated in a number of human neurological and psychiatric diseases, including Parkinson's, schizophrenia and addiction. Biogenic amines are a group of neuromodulators used by all animal brains to regulate the development, structure and function of neural circuits. Although the anatomical characterization and functional significance of biogenic amine projections are understood to a moderate degree, the precise mechanisms by which these molecules exert control over behavior are not fully understood. To decipher the mechanisms by which these molecules exert their influence on the brain and behavior, we must perform sensitive and specific measurements of neuromodulator transients, both broadly (volume modulation) and locally (targeted modulation), with the requisite spatial and temporal resolution, ideally in intac circuits. Existing methods, encompassing microdialysis and cyclic voltammetry, are useful, but not adequate for this task at hand. One potential solution would be to develop genetically encoded indicators based on fluorescent proteins combined with modern microscopy allowing direct and specific measurement of diverse types of neuromodulators with enhanced spatial and temporal resolutions. Recently we have successfully established technology platform for the development of genetically encoded indicators of neural activity, which have led to several high-quality optical probes for simultaneous imaging of large-scale neuronal populations in living animals. Building upon highly optimized platform for sensor sensors and extensive experience in sensor characterization and application in neuroscience, we propose to develop a high-quality toolkit of optical sensors for the biogenic amine neuromodulators, especially for dopamine, the most behavioral pervasive neuromodulator. Our specific aims will start by designing and screening sensors for each of the biogenic amines using combined computational redesign and direct revolution. We will then develop synaptic targeting strategies to display the sensors in dendrites and axons to improve their utility for synaptic imaging. We will finally characterize the performance of these sensors in living neurons and in rat brain slices and demonstrate their capabilities of probing dynamics of dopamine transients in living animals. State-of-the-art sensors for these molecules will facilitate the non-invasive, precise, direct and continual measurement of released neuromodulators at both the synaptic and circuit levels in live model organisms. Such technology advance in optical recordings will facilitate neural circuitry mapping and paint a dynamic picture of neuromodulation systems in regulating neural circuitry and behavior. Given the clear relevance of the biogenic amines to the neurological diseases, these sensors are especially beneficial for long-term studies of human stem cell and animal disease models (specially the Parkinson's disease) and evaluating the effects of candidate therapeutics.